背板对聚甲基丙烯酸甲酯向下火蔓延的影响

Effects of the backboard on downward flame spread over polymethyl methacrylate

聚甲基丙烯酸甲酯(PMMA)是主要的建筑装饰材料之一,一旦被点燃可能会导致严重的火灾事故。目前针对背板对PMMA向下火蔓延影响的实验研究较少。该文利用自主设计搭建的实验装置研究了背板对2mm和5mm厚PMMA向下火蔓延的影响。实验结果表明:背板对固体可燃物向下火蔓延存在抑制作用,具体表现为更大的热解前缘角度及更小的平均火焰高度、质量损失速率和火蔓延速率,该抑制作用在2mm厚度PMMA上更加明显。基于有无背板工况下的传热模型,结合燃烧稳定阶段样品热解区域形态参数,对质量损失速率及接收总热量反馈预测值大小进行对比,发现背板的存在会导致样品接收总热量反馈减少,进而导致更小的质量损失速率和火蔓延速率。

Polymethyl methacrylate(PMMA)is one of the main architectural decoration materials and is widely used in different types of buildings.Once ignited,the flammable PMMA can cause serious building fire accidents.In building decoration,PMMA is used alone or attached to the noninflammable wall.Consequently,the PMMA could burn in a free condition or under the wall effect in a building fire.However,experimental studies on the effect of the backboard on PMMA downward flame spread are rare.In this study,the effect of the backboard was experimentally studied on the downward flame spread for PMMA of 2-and 5-mm thicknesses using a self-designed experimental setup.The experimental setup holds a noninflammable backboard that can be dismantled for the downward flame spread of PMMA in the free condition.Experimental observation showed significant changes in the flame color and pyrolysis front shape with and without the backboard.As compared to the free condition,the airflow induced by the entrainment of the burning flame might cause a flow boundary layer across the backboard,which helped to qualitatively explain the difference in the flame color and the pyrolysis front shape between the two thickness conditions.The experimental measurements also showed that the backboard had a negative effect on the downward flame spread rate,particularly for the 2-mm thick PMMA,which was characterized by an increase in the pyrolysis front angle and a drop in the average flame height,mass loss rate,and flame spread rate.The heat transfer analyses for the steady flame spread under the backboard and free conditions were conducted using the geometrical feature of the pyrolysis zone,and thus a comparison between the two conditions in terms of total heat feedback was conducted.There are four main conclusions.(1)The backboard caused a darker flame of PMMA and a blue flame of the 2-mm thick PMMA,as compared to those in the free condition.(2)Under the backboard effect,the pyrolysis front showed a"-"shape when the 2-mm thick PMMA burned in a steady stage,while the 5-mm thick PMMA held the pyrolysis front of an inverted"V"shape with a larger front angle than that in the free condition.(3)During the steady stage of flame spread,the backboard reduced the mass loss and flame spread rates by decreasing the total heat feedback to the pyrolysis zone.(4)The reductive effect was more significant on the mass loss rate and the flame spread for the 2-mm thick PMMA than the 5-mm thick PMMA because the former could be fully located in the flow boundary layer,whereas the latter partially located in the flow boundary layer in terms of the thickness.In summary,the qualitative and quantitative analyses of the experimental results show that the backboard can slow down the downward flame spread rate by restricting the air entrainment and reducing the heat feedback,and the inhibiting effect is more obvious for the thinner PMMA.